1. Field of the Invention
The present invention relates to a control circuit which controls an image signal for displaying image on a multi-gradation liquid crystal display and a controlling method therefor and, more particularly, to an image signal control circuit which inverts the polarity of an image signal to be applied to liquid crystal at every horizontal synchronization period and a controlling method therefor.
2. Description of the Related Art
In a multi-gradation liquid crystal display, a driving circuit is provided for driving source lines (column line) and gate lines (row line) arranged in a two-dimensional matrix on the display. A source driving circuit for driving a source line supplies a source line with an electric signal corresponding to gradation image data. Then, with a gate driving circuit sequentially selecting and driving a gate line, gradation data is sent to each pixel of the display. This operation is repeated at every selection of each gate line. In a case of such one-line one-scan driving system, a period from the selection of one row to the selection of the next row is referred to as a horizontal synchronization period. A period from when sequential selection of gate lines starts at from the uppermost row to the lowermost row in the two-dimensional matrix on a display panel until when selection of the uppermost row gate line starts again is referred to as a vertical synchronization period or one frame period.
In general, digital image data supplied from an information processor such as a personal computer or the like is subjected to various image processing and then sent to a source driving circuit as analog image data which is analog-gradated by a D/A conversion circuit (digital-to-analog conversion circuit). The analog image data sent to the source driving circuit is sent therefrom as analog pixel data to each pixel through a source line during a horizontal synchronization period. Proposed as a circuit which implements conversion of this pixel data to be written on an arbitrary pixel on a display panel by a source driving circuit from digital data to analog data is such a D/A conversion circuit as shown in FIG. 10.
As illustrated in FIG. 10, the D/A conversion circuit includes a differential input type operation amplifier OP, a first switch SW .alpha. and a capacitor denoted by a capacitance value 2.sup.a.epsilon. C connected in parallel between an output terminal and a negative input terminal of the differential input type operation amplifier OP, and a number a+1 of capacitors, C, 2.sup.0 C, 2.sup.1 C, .about., 2.sup.a-2 C, and 2.sup.a-1 C (C denotes a unit capacitance value) connected in parallel to the negative input terminal of the differential input type operation amplifier OP. A positive input terminal of the differential input type operation amplifier OP is connected to a first reference voltage terminal Vref. Of the above-described capacitors, each of the others of the respective ends of a number a of the capacitors, 2.sup.0 C, 2.sup.1 C, .about., 2.sup.a-2 C and 2.sup.a-1 C is connected to each of ones of the respective ends of a number a of switch groups in which switches SW1-SWa and switches SW1n-SWan are paired respectively. The others of the respective ends of the switches SW1-SWa are connected to a second reference voltage terminal V(m+1). The others of the respective ends of the switches SW1n-Swan are connected to a third reference voltage terminal Vm through a switch SW.beta. as well as to the first reference voltage terminal Vref through a switch SW.beta.n. In addition, the capacitor with the capacitance value C is connected to the third reference voltage terminal Vm through the switch SW.beta. as well as to the first reference voltage terminal Vref through the switch SW.beta.n.
A thus structured conventional D/A conversion circuit selects two values out of 8 to 10 gradations of analog gradation voltages whose .gamma. correction has been already completed outside a driving circuit according to optical characteristics of liquid crystal and applies the selected two values of analog gradation voltages to the reference terminals Vm and V(m+1). Responsively, selective operation of the first to third switches and a number a of switch groups in a not-shown D/A converter control circuit results in multi-division of the analog gradation voltage is to multi-gradate gradation data. Then, one value of the multi-gradated gradation data is output as analog image data and the image data is supplied to a pixel through a source line.
Common liquid crystal displays perform displaying while inverting the polarity to be applied to liquid crystal at every one horizontal synchronization period in order to improve image quality. Conventional driving techniques using the D/A conversion circuit shown in FIG. 10 accordingly need to invert the polarity of 8 to 10 gradations of analog voltage values whose .gamma. correction has been conducted at every one horizontal synchronization period. It is therefore necessary to invert the polarity of a voltage value to be supplied to the reference voltage terminals Vm, and V(m+1).
Inverting the polarity of a voltage value supplied from the outside of a driving circuit, however, is a great burden to a liquid crystal driving system and requires large power consumption.